1. Field
One embodiment of the present invention relates to an optical disc device and an optical disc playback method, and more particularly, to an optical disc drive and an optical disc playback method for decoding recoded data by sampling a playback signal of an optical disc as multilevel data.
2. Description of the Related Art
In the field of optical discs, in particular, high-density optical discs, a playback system called Partial Response Maximum Likelihood (PRML) signal processing system is commonly used. The PRML signal processing system is based on combination of a narrowband response characteristic (PR) tolerating unintentional waveform interference and a maximum likelihood (ML), as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-93933. Noise components are increased according to density increasing of optical discs and the S/N ratio of a playback signal decreases. In spite of such a low S/N ratio, error detection can be greatly decreased by using the PRML signal processing system as compared to the conventional binary data slice system.
In the PRML signal processing system, an analog playback signal is sampled as multilevel data, and amplitude information of the playback signal is aggressively used for data decoding. This makes the PRML signal processing system greatly different from the conventional binary slice system using thresholds.
In a circuit in which an analog filter limiting the bandwidth of an analog playback signal obtained from the optical disc is manufactured by a CMOS process, variation in manufacturing process and change in characteristics due to temperature change are considerably great. Therefore, the transfer characteristics of the overall filter may change greatly even if the analog filter is set with the same cutoff and the same boost amount.
It is therefore desirable to set the cutoff and the boost amount (high-frequency amplification amount) in consideration of variation in manufacturing process of the analog filter and change characteristics due to temperature change. An analog filter has previously been adjusted utilizing spectrum analysis based on a band pass filter, for example (see Jpn. Pat. Appln. KOKAI Publication No. 1-220268).
In the above-described method in which an analog filter is adjusted utilizing a band bass filter, adjustment accuracy is low, and such a method is difficult for application to a playback device which reproduces a high-density optical disc. Further, when a separate circuit for preventing variation in characteristics is provided in an analog filter itself, the manufacturing cost may increase.
The present invention has been made in consideration of the above-described circumstances, and is capable of providing an optical disc device and an optical disc playback method with reduced costs and an improved reading rate.